Electrical power distribution grids use electrical components, such as transformers, capacitors, and reactors. Potentially dangerous conditions can be created in such devices when aging or operating stresses cause the insulation system to fail. A short circuit within such a device can release a large amount of energy within a fraction of a second. In the worst case the device can explode due to rapid internal pressure buildup from the vaporization of insulating oil and the decomposition of the oil vapor into combustible or volatile gases.
Nearly all pole mounted distribution transformers are protected by a cutout which includes an expulsion fuse, current limiting fuse, or some other fast acting protective device. Such cutouts can minimize damage by disconnecting a faulty device from its source of electric energy so as to interrupt arc current in the event of an overload or internal fault. Service personnel can also use cutouts as manual switches for energizing or disconnecting particular circuits. If there is an overload in the system and the cutout operates, then service personnel can easily spot the open cutout and know that the transformer disconnected by the open cutout is out of service. If the fault is downstream of the transformer then, once that fault has been corrected, it is a simple matter for service personnel to re-fuse the cutout to re-energize the circuit. Similar types of protective fuses are also used in voltage regulators, and also in pad-mount transformers, where, for example, a bay-o-net fuse or current limiting fuse may be provided.
If the fault is in the transformer then closing the cutout before the transformer has been repaired will likely produce arcing within the transformer. A device that has failed once is certain to fail again if it is re-energized before the internal damage caused by the arcing has been corrected. Arcing can leave carbonized paths within the device and may impair the mechanical integrity of the device's housing, or “tank”. This increases the risk that the device will fail catastrophically if it is re-energized. In extreme cases the transformer may explode. This could cause property damage and serious injury to service personnel and any members of the public who happen to be close by. To avoid this possibility service personnel must perform careful inspections and take special precautionary measures before attempting to re-energize any electrical apparatus found disconnected from the power system by its protective device.
Unfortunately, an internal fault can occur in a device without leaving any obvious visible cues that the fault has occurred in that device. Unless service personnel can tell that a particular device has failed they may reapply power to the device without detecting that the electrical device has failed. This may cause catastrophic failure of the electrical device, as noted above.
It is known that there is a transient or rapid surge in pressure inside oil-filled electrical devices, such as transformers or voltage regulators, when the devices suffer from an internal arcing fault. This happens because arcing produces a marked increase in temperature which vaporizes some of the oil. Some electrical devices are filled with electrically insulating gases such as SF6. In such gas-filled devices arcing causes pressure surges in the gas. It is desirable to provide a device capable of detecting such transient pressure surges.
Space is at a premium inside the housing of a typical electrical device. This is especially the case in the top portion of the electrical device. A pressure relief valve and an internal fault indicator should both be in the air space at the top of the housing. In typical electrical devices, power leads also enter the housing through the upper air space region.
Additionally, it is important that the interior of a transformer or voltage regulator remain sealed from the outside atmosphere at all times during transportation and normal usage. The electrically insulating fluid or gas used inside such transformers may pose an environmental concern if permitted to leak from the device.
Documents of interest relating to indicators for detecting a transient pressure surge in an electrical device include U.S. Pat. No. 6,812,713 to Cuk et al., U.S. Pat. No. 6,429,662 to Cuk et al., and U.S. Pat. No. 5,078,078 to Cuk, each of which is hereby incorporated by reference herein.
In view of the foregoing, there remains a need for an internal fault indicator that can be readily installed and used with an electrical component with a minimum risk of being incorrectly installed or deployed, and further which remains sealed at all times during normal transportation and use of the electrical device, including after deployment of the indicator.